Is it possible to make a POST as being on A client side using google app script?

Question

As a title, I am currently working on a project that requires me to connect to a third-party payment API with Google App Script. However, after all the effort, the tech support says, that I have to make a CLIENT POST, rather than a SERVER POST. Therefore it is possible to make a post request as a client using google app script. The following is the script I currently have.

function ecpay_payment()
{
  var payment_Object = {payment_data:[]};
  payment_Object.payment_data.push({
        'ChoosePayment':'CVS',
        'EncryptType':"1"  ,
        'ItemName':'Test',
        'MerchantID':'2000132',
        'MerchantTradeDate':Utilities.formatDate(new Date(), "GMT+8", "yyyy/MM/dd HH:mm:ss"),
        'MerchantTradeNo':'20210809002',
        'PaymentType':'aio',
        'ReturnURL':'https://script.google.com/macros/s/AKfycT5khFUkTbmLJWJXP9Y6sovFRJig/exec',
        'TotalAmount':500,
        'TradeDesc':'WriteAwayPLUS'
    })       
    //'CheckMacValue':macvalue,
   //JSON字串化(STRING) 
   var payment  = JSON.parse(JSON.stringify(payment_Object.payment_data[0]));
   var qs ;
   for(var key in payment ) {
      var value = payment[key];
      qs +=   key  + "=" +  value  + "&";
  }
   payment = qs.replace('undefined', '', );
   //字串加KEY
   payment = "Hashkey=5294y06JbISpM5x9&" + payment + "HashIV=v77hoKGq4kWxNNIS";
   console.log(payment);// Success
   //文字ENCODE
   var payment= encodeURIComponent(payment);
   //ENCODE不同點修正
   payment = payment.replace('%7e', '~', );
   payment = payment.replace('%27', "'", );
   payment = payment.replace('%20', "+", );
   //全文轉小寫
   payment = payment.toLowerCase();
   //加密
   Hashvalue= Sha256Hash(payment);
   //密文內容轉大寫
   var macvalue = Hashvalue.toUpperCase();
   //密文加入JSON
   var payment_json = JSON.parse(JSON.stringify(payment_Object.payment_data[0]));
   payment_json.CheckMacValue = macvalue;

   var options = {
      'method' : 'POST',
      'contentType': 'application/x-www-form-urlencoded',
      'payload' : payment_json 
   };
   var resp= UrlFetchApp.fetch("https://payment-stage.ecpay.com.tw/Cashier/AioCheckOut/V5", options);
          var code = resp.getResponseCode();
           if (code >= 200 && code < 300) 
           {
              //var ret = JSON.parse(resp.getContentText("utf-8"));
              console.log(resp.getContentText("utf-8"));// Success
           } 
           else if (code == 401 || code == 403)
           {
             maybeAuthorized = false;
             return "error";
           } else 
           {
               console.error("Backend server error (%s): %s", code.toString(),resp.getContentText("utf-8"));
               return "error";
           }           



    //console.log(t);
    var tempText = t.toLowerCase();
    //tempText= tempText.toLowerCase();
    //console.log(tempText);
    tempText= Sha256Hash(tempText);
    var macvalue = tempText.toUpperCase();
    console.log(macvalue);
}


function formatDate(){
//= (current_datetime)=>{
    var formatted_date= new Date();
var yyyy = formatted_date.getFullYear(); 
var MM = (formatted_date.getMonth()+1<10 ? '0' : '')+(formatted_date.getMonth()+1);
var dd = (formatted_date.getDate()<10 ? '0' : '')+formatted_date.getDate();
var h = (formatted_date.getHours()<10 ? '0' : '')+formatted_date.getHours();
var m = (formatted_date.getMinutes()<10 ? '0' : '')+formatted_date.getMinutes();
var s = (formatted_date.getSeconds()<10 ? '0' : '')+formatted_date.getSeconds(); 
     formatted_date= (yyyy+"/"+MM+"/"+dd+" "+h+":"+m+":"+s);
    return formatted_date;
}

Answer 1

What is that POST supposed to accomplish? If you want to redirect the user as if they clicked a form, make a <form> and trigger the submit action on it with JavaScript or a clickable button.

If you want to stay on the current page and just do an asynchronous/XHR request from the browser (client), use the JavaScript Fetch API

If you want to communicate with that server without a browser, their support is telling you not to do that as whatever endpoint you're communicating with is not designed for that. Doing a CLIENT post requires a browser user agent (the client) to actually make the post.